The present disclosure relates generally to methods and apparatus for sensing temperature proximate a drill string tool conveyed in a borehole. The present disclosure relates more particularly to methods and apparatus for sensing the temperature of drilling fluid in the inner diameter, or flowbore, of the drill string tool or in the annulus between the outer diameter of the drill string tool and the borehole.
To recover hydrocarbons from subterranean formations, wells are generally constructed by drilling into the formation using a rotating drill bit attached to the lower end of an assembly of drill pipe sections connected end-to-end to form a drill string. In some cases the drill string and bit are rotated by a drilling table at the surface, and in other cases the drill bit may be rotated by a downhole motor within the drill string above the bit, while remaining portions of the drill string remain stationary. In most cases, the downhole motor is a progressive cavity motor that derives power from drilling fluid (sometimes referred to as mud) pumped from the surface, through the drill string, and then through the motor (hence the motor may also be referred to as a mud motor).
Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes borehole environmental information, such as temperature, pressure, etc., and drill string operational information. Temperature is a common downhole reading; however, sensors are often not placed optimally for temperature measurements. Sensors are typically disposed on the downhole tools and measure the temperature of the tool housing and do not track temperature changes very well. Alternatively, temperature sensors may be placed at the point of interest; however, the point of interest in a borehole is in the path of the fluid flowing either through the internal diameter (ID) of the drill pipe or through the annulus formed about the outer diameter (OD) of the pipe. In either case, an exposed temperature probe is difficult to handle and subject to erosion from the fluid flowing at hundreds of gallons per minute (GPM).
There is a need to measure small temperature changes in the borehole while drilling. Temperature changes on the order of tenths of a degree are very informative of the borehole environment and provide a method for predicting the events that will follow. Temperature has an impact on all downhole readings and being able to detect small changes in temperature allows the exact temperature coefficient in every calculation be determined, which helps correctly depict the temperature reading by subtracting the temperature effects from other readings. However, commonly used temperature measuring systems can be inaccurate due to a margin of error from +/−2° C. up to +/−5° C. at higher temperatures, non-optimal sensor positioning as previously discussed, temperature dissipation in the body in which the housing of the downhole tools acts as a shield against rapid temperature changes and delays the sensor's ability to detect rapid temperature changes, and low precision of the temperature sensor where the sensor resolution is limited to 1.0 or 0.5° C. There is a further need to prevent drilling fluid and cuttings from becoming packed around the temperature sensors. Drilling fluid acts as a thermal insulator and may prevent true temperature measurement readings as the temperature fluctuates.